Such an extrusion device with an extrusion head is needed, for example, for the production of a tread for car tires. Nowadays, such tires are already made of four or more different mixtures bonded to each other, namely, the tread mixture, which consists of a highly abrasion-resistant material, the shoulder strip mixture, which consists of very millable material, the so-called rubber base mixture that serves as the substrate for the tread and that can be made of a softer mixture than the tread, and the so-called cushion-rubber mixture for good adhesion to the tire body. Therefore, four or more mixtures with very different viscosity values have to merge in one extrusion head to form the tread profile.
Electrostatic charges built up in vehicles not only cause electric discharges but also impair vehicle functions owing to possible spark formation. Such problems do not arise if the tire is made of rubber containing a large amount of carbon black since this causes the tire to be sufficiently conductive.
For financial reasons as well as for environmental considerations, there is a rising demand for tires that can reduce the fuel consumption of vehicles.
In tires that have a low rolling resistance, however, a considerable amount of the carbon black is replaced by silica, which increases the electric resistance of the tread rubber.
German patent specification DE 195 20 996 C2 discloses, for instance, a car tire with a tread made of electrically insulating or low-conductivity material, with a highly electroconductive layer that is located underneath the tread, with a strip of highly electroconductive material that forms part of the tread surface, and with a layer of highly electroconductive material arranged between the two abutting faces of the ring-shaped tread. As seen over the circumference of the tire, the highly electroconductive layer extends at least partially into the tire tread and is electrically connected to another highly electroconductive tire component which, in turn, is connected to the car body in a highly electroconductive manner.
Furthermore, European patent application EP 1 060 089 B1 also discloses a tire with a multilayered structure.
Tire components consisting of up to four different mixtures are produced in a single operation employing multiple extrusion heads. These aggregates—which are equipped with two, three or even more extruders mounted above one another and with matching extrusion heads—have replaced the older variants (boxer or Y-design arrangements). Arranging the extruders above one another in a single axis also allows a symmetrical shape of the flow channels. The channels have bores for sensors to measure the compound pressure and the temperature. The flow channel inserts are screwed into the lower part of the head so that they can be replaced quickly.
European patent application EP 0 925 903 A1 relates to a method and to a device for the production of a pneumatic tire comprising a tread rubber that can discharge electrostatic charges to the road surface. A device for the production of a non-vulcanized tread rubber is proposed, whereby at least one low-conductivity rubber and at least one highly conductive rubber are each fed to an extruder unit located upstream from a shared extrusion head. The rubber types flow through individual passages in the extrusion head and are united with each other at the rear of the extrusion die, before they are extruded out of the extrusion die as an integrated composite.
European patent application EP 1 270 174 A1 relates to an extrusion die for tire treads, comprising an outer tread passage to form an electrically non-conductive rubber outer tread as well as an inner tread passage to form an inner layer made up of an electroconductive rubber.
British patent GB 2 131 734 A discloses an extruder installation for creating flat profiles consisting of two different mixtures. The extruder head of the installation is split over two planes arranged above each other. The mid-part of the head is firmly joined to the extruders, while the upper part and the lower part are configured so that they can be swiveled.
In many operations, a profile strip change is performed relatively often, for instance, in order to produce treads for car and truck tires of different sizes or for special tires for earth-moving equipment.
Each change in the type of tread means that the flow channel inserts as well as the pre-forming bars also have to replaced.